1. Field of the Invention
The present invention relates to a method of forming a T-shaped pattern and more particularly, to a method of forming a T-shaped, cross-sectional pattern using two layered masks, which is applicable for producing a gate electrode of a field-effect transistor that has a T-shaped, cross-section using the lift-off technique.
2. Description of the Prior Art
A conventional method of forming a T-shaped, cross-sectional pattern using upper and lower layered masks is shown in FIGS. 1A to 1G, which is disclosed in the Japanese Patent Publication No. 4-368135 (December, 1992). In the method, a positive photoresist film is used as an upper one of the layered mask and a positive electron resist film is used as a lower one thereof.
First, as shown in FIG. 1A, an electron resist film 22 as the lower mask is formed on a semiconductor substrate 21 and then, a photoresist film 23 as the upper mask is formed on the electron resist film 22 to cover the film 22.
Next, as shown FIG. 1B, the photoresist film 23 is selectively exposed to ultraviolet (UV) light 24 to transfer a given pattern to the film 23.
Using a known image-reversal technique, the exposed portion 23b of the photoresist film 23 is changed to be less soluble and the unexposed portion 23a thereof is changed to more soluble in the developer solution.
Subsequently, the photoresist film 23 thus image-reversed is developed, so that the unexposed part 23a of the film 23 is selectively removed to produce a window 28 with a tapered profile therein, as shown in FIG. 1C. In the tapered window 28, the top end of the film 23 overhangs the bottom end thereof; in other words, the film 23 is undercut. The electron resist film 22 is not covered with the photoresist film 23 in the window 28.
Then, as shown in FIG. 1D, an electron beam (EB) 25 is selectively irradiated to the electron resist film 22 through the window 28. The irradiated part 22a of the film 22 becomes more soluble in a developer solution for the film 22 due to interaction of electrons and the film material.
The part 22a of the electron resist film 22 is selectively removed through a development process to produce a window 29 in the film 22, as shown in FIG. 1E. Thus, a pattern with a T-shaped, cross section is formed by the windows 28 and 29 on the substrate 21.
Subsequently, the substrate 21 is selectively etched through the windows 28 and 29 to form a recess 26 with a given depth therein. Then, to form a gate electrode of a field-effect transistor using the T-shaped pattern, a metal film 27 is formed by evaporation on the photoresist film 23, so that the metal film 27 is deposited also on the electron resist film 22 through the window 28 and on the substrate 21 through the windows 28 and 29, as shown in FIG. 1F.
Finally, both of the photoresist film 23 and electron resist film 22 are removed by the lift-off technique. As a result, only a part of the metal film 27 within the windows 28 and 28 is left, providing a gate electrode with a T-shaped cross section as shown in FIG. 1G.
As another conventional method, a method of forming a fine pattern applicable for semiconductor device fabrication is disclosed in the Japanese Patent Publication No. 5-166717 (July, 1993).
In this conventional method, a first resist film is formed on a semiconductor substrate and is patterned to produce a window therein. Then, a second resist film capable of producing a mixing layer is formed on the first resist film with the window.
The first and second resist films are baked to produce a mixing layer at the interface of the both films and thereafter, the second resist film is removed except for the mixing layer that is not yet subjected to the mixing phenomenon.
Thus, a window smaller than the initial window of the first resist film by twice the thickness of the mixing layer can be obtained.
With the conventional method shown in FIGS. 1A to 1G, the window 28 is formed in the photoresist film 23 as the upper mask, and then the window 29 is formed in the electron film 22 as the lower mask. As a result, there is a disadvantage that the window 29 is impossible to be formed in the electron resist film 22 by using the developer solution for the film 22 if the mixing layer is produced at the interface of the films 22 and 23.
Also, since the minimum size or limit of the window 29 is determined by the performance of an electron beam lithography system used for the irradiation process of FIG. 1D, there arises another disadvantage that the windows 29 cannot be made less than the limit.
There is a further disadvantage that the electron resist film 22 as the lower mask cannot be developed by using any developer solution that dissolves the photoresist film 23 because the electron resist film 22 is electron-irradiated and developed after forming the window 28 in the photoresist film 23.